A legged robot can walk and is good at travelling in the human living environment including uneven terrain, and so the basic technology therefor has been researched actively. In spite of the competition for such active researches, the technique for controlling a legged robot and its hardware technique have a limit, and few legged robots in practical use exist. Technical problems of the legged robots are that the robots have a large size and heavy weight, the cost is high, the moving speed is low, and the robots may fall, for example. Many of these problems result from a problem in the control technique or the robot having a lot of actuators. The balance of a conventional legged robot often is controlled based on ZMP (Zero Moment Point). However, if the ZMP is outside of the plane including the soles of the both feet, the heel or the toe of the robot will float. Then the toe or the heel on the side in contact with the ground has a state equal to a joint without actuator.
Few effective methods for controlling the balance in that case have been proposed. In many conventional techniques to control robots, the robot has two actuators at their ankles to control its total freedom. Such two actuators attached to the end of each leg, however, increase in size and weight of the end of the legs and increase in cost. Moving of the end of the legs requires actuators at the knees and the hip joint also having higher output. This means an increase in size of the knees and a part around the hip and an increase in weight and cost. As a result, the robot as a whole increases in size and weight, increases in cost, and decreases in moving speed. Legged robots without actuators at the ankles also have been researched, although fewer in number. Due to such researches, control algorithm enabling the gait of a robot at a certain speed is being established. This can eliminate the four actuators in total at the ankles of both legs, and so can contribute to downsizing, lighter weight, lower cost, and higher moving speed of the robot.
Keeping a certain upright posture of a robot without actuators at the ankles is essential to realize a legged robot. However, the technique for controlling a robot to keep a certain upright posture is extremely insufficient conventionally. This is an issue to be addressed.
Patent Literature 1 describes a method for controlling the posture of an articulated robot. This method enables the robot to return to its original upright posture after a small external force acts on the robot. Non Patent Literature 1 describes a technique relating to a legged robot with actuators at the ankles. The technique specifies a maximum range of enabling the robot to return to its original posture using the torque at ankles when the robot is pushed by an external force or the like.